This invention pertains generally to the ignition of solid rocket propellant and more particularly to the initiator that produces an intermediate combustion step in a system for igniting propellant with a laser beam.
The conventional intermediate combustion source in a system for igniting a solid propellant with a laser beam is a round pyrotechnic pellet that is flat on top and bottom. One of the flat sides of the pellet is placed against a window in a container that holds the pellet and a laser beam is directed through the window onto the pellet, thereby igniting the pellet. The combustion products from the pellet initiate combustion of other materials if an igniter which in turn produce other combustion products which flow from the igniter onto the main mass of propellant being ignited. An alternative to this last step is for the combustion products from the pellet to flow directly onto the main mass of propellant being ignited.
The flat sided pellet used in this system has been found to produce undesirable results. Burning rate cannot be controlled within the desired limits and pellets with highly reflective surfaces may not absorb sufficient energy from the laser beam to initiate combustion within the desirable time of 15 microseconds. Pellets of fast burning pyrotechnic material produce high initial pressure between the pellet and the laser entrance window that can cause the window to break. If a thicker window is used in an attempt to overcome this breakage, the additional thickness attenuates the laser energy that gets to the pellet, thus reducing the efficiency of combustion initiation by the beam. When the window is thick enough to successfully resist breaking, the high initial pressure between the window and the pellet surface can cause the uncombusted portion of the pellet to be fractured and blown from its container. The flying pieces of pellet can damage the igniter or motor, and adversely affect ignition efficiency. The sudden drop in pressure that occurs because of the fracture of the pellet can cause combustion to be extinguished.
Application of the present invention in igniting solid propellants with laser beams has demonstrated that the invention overcomes all of the aforementioned shortcomings of the flat sided pellet.